On-line coupling of high-resolution NMR

The major advantage of on-line NMR detection as opposed to off-line detection for liquid chromatography (LC) are improved chromatographic resolution, consistent response, and rapid data acquisition. The disadvantages include poorer sensitivity due to limited measurement time and a flow-rate dependence of the NMR linewidth [Webl]. Following the first stopped flow design in 1978 [Wat 1 ], an on-line system had been developed within a year [Bayl], and the techniques have been improved continuously since then [Webl]. 

The standard LC-NMR detector is a saddle coil which is wound on the flow cell with a diameter of 2-4 mm and a length of 12-14 mm (Fig. 9.3.5) [Webl], The cell consists of a glass tube. Inflow and outflow are provided by tubings from FIFE with an inner diameter of typically 0.25 mm. The associated chromatography column is positioned outside the magnet. Because of time requirements 2D-NMR spectra can be acquired only in stopped flow mode, while in continuous flow mode ID-NMR spectra are acquired as a function of the elution time. 

In continuous flow mode the relaxation times are affected by the ow rate and may be expressed in terms of an effective residence time t within the flow cell. 

Increasing the flow rate decreases the effective longitudinal relaxation time, which allows shorter repetition delays Ir. However, at the same time the effective transverse relaxation rate increases, leading to line broadening. 

As an example of a continuous flow chromatogram Fig. 9.3.6 shows a stack of NMR spectra (right) as a function of elution time for five vitamin A acetate isomers on a cyanopropyl column in n-heptane recorded at a flow rate of 0.2 ml/min [Albl]. The first two peaks somewhat overlap in the ultraviolet (UV) detected chromatogram (left), but in the NMR spectra (right) the peaks are well separated due to different chemical shifts. Forty-eight scans were added for each trace, so that the time resolution was 62 s. 

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